Modern computing and display technologies have facilitated the development of systems for so-called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user and may be perceived as real. A virtual reality (“VR”) scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input. An augmented reality (“AR”) scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. For example, referring to FIG. 1, an augmented reality scene 100 is depicted wherein a user of an AR technology device sees a real-world park-like setting 102 featuring people, trees, buildings in the background, and a concrete platform 104. In addition to these items, the user of the AR technology also perceives that he/she “sees” a robot statue 106 standing upon the real-world platform 104, and a cartoon-like avatar character 108 flying by, even though these elements (106, 108) do not exist in the real world. As it turns out, the human visual perception system is very complex, and producing a VR or AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements is challenging.
Referring to FIG. 2, stereoscopic wearable glasses 200 type configurations have been developed which generally feature two displays (e.g., 202, 204) that are configured to display images with slightly different element presentation such that a three-dimensional perspective is perceived by the human visual system. Such configurations have been found to be uncomfortable for many users due to a mismatch between vergence and accommodation that must be overcome to perceive the images in three dimensions. Indeed, some users are not able to tolerate stereoscopic configurations.
Referring to FIG. 3, a simplified cross-sectional view of a human eye 300 is depicted featuring a cornea 302, iris 304, lens—or “crystalline lens” 306, sclera 308, choroid layer 310, macula 312, retina 314, and optic nerve pathway 316 to the brain. The macula is the center of the retina, which is utilized to see moderate detail. At the center of the macula is the “fovea”, which is used for seeing the finest details. The fovea contains more photoreceptors (approximately 120 cones per visual degree) than any other portion of the retina.
The human visual system is not a passive sensor type of system. It is configured to actively scan the environment. In a manner somewhat akin to scanning an image with a flatbed scanner or using a finger to read Braille from a paper, the photoreceptors of the eye fire in response to changes in stimulation, rather than constantly responding to a constant state of stimulation. Indeed, experiments with substances such as cobra venom, which is utilized to paralyze the muscles of the eye, have shown that a human subject will experience blindness if positioned with his/her eyes open, viewing a static scene with venom-induced paralysis of the eyes. In other words, without changes in stimulation, the photoreceptors don't provide input to the brain and blindness is experienced. It is believed that this is at least one reason that the eyes of normal humans have been observed to move back and forth, or dither, in side-to-side motion in what are called “microsaccades”. As noted above, the fovea of the retina contains the greatest density of photoreceptors, and while humans typically have the perception that they have high-resolution visualization capabilities throughout their field of view, they generally actually have only a small high-resolution center that they are mechanically sweeping around a lot, along with a persistent memory of the high-resolution information recently captured with the fovea. In a somewhat similar manner, the focal distance control mechanism of the eye (ciliary muscles operatively coupled to the crystalline lens in a manner wherein ciliary relaxation causes taut ciliary connective fibers to flatten out the lens for more distant focal lengths; ciliary contraction causes loose ciliary connective fibers, which allow the lens to assume a more rounded geometry for more close-in focal lengths) dithers back and forth by approximately ¼ to ½ diopter to cyclically induce a small amount of what is called “dioptric blur” on both the close side and far side of the targeted focal length. This is utilized by the accommodation control circuits of the brain as cyclical negative feedback that helps to constantly correct course and keep the retinal image of a fixated object approximately in focus.
The visualization center of the brain also gains valuable perception information from the motion of both eyes and components thereof relative to each other. Vergence movements (i.e., rolling movements of the pupils toward or away from each other to converge the lines of sight of the eyes to fixate upon an object) of the two eyes relative to each other are closely associated with focusing (or “accommodation”) of the lenses of the eyes. Under normal conditions, changing the focus of the lenses of the eyes, or accommodating the eyes, to focus upon an object at a different distance will automatically cause a matching change in vergence to the same distance, under a relationship known as the “accommodation-vergence reflex.” Likewise, a change in vergence will trigger a matching change in accommodation, under normal conditions. Working against this reflex, as do most conventional stereoscopic AR or VR configurations, is known to produce eye fatigue, headaches, or other forms of discomfort in users.
Movement of the head, which houses the eyes, also has a key impact upon visualization of objects. Humans move their heads to visualize the world around them. They often are in a fairly constant state of repositioning and reorienting the head relative to an object of interest. Further, most people prefer to move their heads when their eye gaze needs to move more than about 20 degrees off center to focus on a particular object (i.e., people don't typically like to look at things “from the corner of the eye”). Humans also typically scan or move their heads in relation to sounds—to improve audio signal capture and utilize the geometry of the ears relative to the head. The human visual system gains powerful depth cues from what is called “head motion parallax”, which is related to the relative motion of objects at different distances as a function of head motion and eye vergence distance (i.e., if a person moves his head from side to side and maintains fixation on an object, items farther out from that object will move in the same direction as the head; items in front of that object will move opposite the head motion. These are very salient cues for where things are spatially in the environment relative to the person—perhaps as powerful as stereopsis). Head motion also is utilized to look around objects, of course.
Further, head and eye motion are coordinated with the “vestibulo-ocular reflex”, which stabilizes image information relative to the retina during head rotations, thus keeping the object image information approximately centered on the retina. In response to a head rotation, the eyes are reflexively and proportionately rotated in the opposite direction to maintain stable fixation on an object. As a result of this compensatory relationship, many humans can read a book while shaking their head back and forth (interestingly, if the book is panned back and forth at the same speed with the head approximately stationary, the same generally is not true—the person is not likely to be able to read the moving book; the vestibulo-ocular reflex is one of head and eye motion coordination, generally not developed for hand motion). This paradigm may be important for augmented reality systems, because head motions of the user may be associated relatively directly with eye motions, and the system preferably will be ready to work with this relationship.
Indeed, given these various relationships, when placing digital content (e.g., 3-D content such as a virtual chandelier object presented to augment a real-world view of a room; or 2-D content such as a planar/flat virtual oil painting object presented to augment a real-world view of a room), design choices may be made to control behavior of the objects. For example, the 2-D oil painting object may be head-centric, in which case the object moves around along with the user's head (e.g., as in a GoogleGlass approach); or the object may be world-centric, in which case it may be presented as though it is part of the real world coordinate system, so that the user may move his head or eyes without moving the position of the object relative to the real world.
Thus when placing virtual content into the augmented reality world, how the content is presented must be given consideration. For example, in a world centric scheme the virtual object stays in position in the real world so that the user may move his/her ahead around it to see the object from different points of view.
The systems and techniques described herein are configured to work with the visual configuration of the typical human to address these challenges.